A large number of solid grain gas generators are produced for use on current missile control systems and aircraft starter turbines. Currently, the aircraft jet engine starter cartridge is the high quantity production item; however, almost every type of missile uses gas generators for various functions. Gas generators are required on numerous propellant actuated ballistic devices. A long felt need exists in the use of a relatively cool gas to inflate life rafts and similar devices; at present there is no completely satisfactory method of inflation.
Gas generators should evolve cool, clean inert gases in a reproducible manner suitable for driving turbines for secondary power devices and for gas servo systems. Most present day gas generators have flame temperatures of over 2000.degree. F., (1093.degree. C) and evolve gases containing solid particles which corrode and erode turbine blades and other mechanical hardware.
The usual gas generator composition, known in gas generator technology as the "propellant," is composed of ammonium nitrate oxidizer with a rubbery binder. Various chemicals ("ballistic modifiers") such as guanidine nitrate, oxamide and melamine are used in the propellant to aid ignition, give smooth burning, modify burning rates and give lower flame temperatures. Ballistic modifiers, such as sodium barbiturate, are used to reduce the temperature sensitivity of the propellant in order to give relatively constant burning rates with changes in temperature and pressure. Lithium oxalate may be used in place of the sodium barbiturate which produces erosive exhaust products. Carbon black is added to give more smooth, stable burning.
Ammonium nitrate is the most commonly used oxidizer since it gives maximum gas horsepower per unit weight and yields a non-toxic and non-corrosive exhaust at low flame temperatures. Further, it contributes to burning rates lower than those of other oxidizers. Ammonium nitrate is cheap, readily available and safe to handle. The main objection to ammonium nitrate is that it undergoes various phase changes during temperature changes causing cracks and voids if the binder is not strong and flexible enough to hold the grain together.
Ammonium nitrate compositiions are hygroscopic and difficult to ignite, particularly if small amounts of moisture have been absorbed. Since they do not sustain combustion at low pressures, various combustion catalysts are added to promote ignition and low pressure combustion as well as to achieve smooth, stable burning. Gas generator compositions used for driving turbines should contain no metallic additives or even oxidizers such as ammonium perchlorate since these give erosive and corrosive exhaust gases respectively.
Commonly used ballistic additives such as ammonium dichromate, copper chromite, Milori blue, carbon black, etc., are disadvantageous since they all produce solids in the exhaust gases.
Gas generator compositions are manufactured by pressing or extrusion and compression molding techniques. The solid particles are milled with a rubbery binder such as cellulose acetate, the solid "C" rubber type or polybutadiene-vinylpyridine and mixed under vacuum at temperatures of 170.degree.-200.degree. F (77.degree.-93.degree. C). After mixing, the composition is broken up into bits ("granulated") with granulator type equipment or cutting type grinders such as the Wiley mill. This is an extremely hazardous operation and must be done remotely.
After granulation, the composition is loaded into molds of the required shapes and pressed to about 7000 psi. (4921 Kg/Sq.cm.) With certain types of binder, the molds are heated to about 180.degree. F (82.degree. C) until the composition is cured or vulcanized. The grain is then machined to size and potted into the gas generator cases. The molds, mills and extrusion equipment are costly; the lengthy process time further increases the cost of manufacture. It is especially difficult to produce large grains by this technique.
The castable case-bonded system which is the standard and preferred method of producing large solid rocket propellant grains would result in tremendous savings to the gas generator producer since the need for expensive compression molding equipment would be eliminated. The main problem is producing castable gas generators in a manner similar to solid rocket propellants is that ammonium nitrate has a relatively low density (1.73 g/ml) as compared to ammonium perchlorate (1.95 g/ml) or aluminum (2.7 g/ml) and this property, coupled with the pourous nature of the crystals, requires high binder levels for castable compositions. These high binder levels (25-30%) result in gas generators which emit excessive quantities of smoke. Also, water may be absorbed from the air by the ammonium nitrate if conventional propellant processing techniques are used.